Method of treating galvanized sheet to inhibit staining



United States Patent 3,271,284 METHOD OF TREATING GALVANIZED SHEET TO INHIBIT STAINING Paul R. Carter, Monroeville Borough, and Lawrence E. Helwig, Hampton Township, Allegheny County, Pa.,

assignors to United States Steel Corporation, a corporation of Delaware No Drawing. Filed Oct. 29, 1963, Ser. No. 319,659

1 Claim. (Cl. 204-145) This invention relates to the treatment of galvanized sheet steel to inhibit staining thereof in storage.

Galvanized steel sheets as now produced are given a chromate surface treatment to inhibit staining or discoloration of the sheets in storage under a humid atmosphere. Attempts have been made to improve the effectiveness of such treatment by a preliminary activation or removal of surface oxide from the sheets by dipping them in a dilute acid bath.

We have discovered that removal of the surface oxide from galvanized sheets by cathodic electrolytic cleaning in tri-sodium phosphate not only improves the effectiveness of the subsequent chromate treatment but also gives rise to other unexpected results, including improved solderability. The cathodic electrolytic cleaning makes the surface of the sheets more active during subsequent treatment than pretreatments which have been used previously, and makes possible the formation of a heavier chromate film by the conventional treatment, which increases the resistance to staining in humid storage.

More particularly, our invention comprises the electrolysis of galvanized sheet steel as cathode in a suitable solution such as a water solution containing from 2 to 10% (by weight) of tri-sodiurn phosphate, or a mixed water solution of 3% tri-sodiurn phosphate, 3% sodium carbonate and 1.5% sodium hydroxide (all by weight). Other electrolytes may be used instead, however, which do not form insoluble reaction products on the sheets or noticeably change their appearance. A minimum electric-current flow of about 300 coulombs per square foot of surface is required to impart the desired activation.

Our invention may be more fully understood from the following description of a few typical examples thereof.

Example 1 TABLE I Reducing Solution Reducing Steam Current, Pressure coulombs Test, cycles Electrolyte Cone, per sq. ft. to failure 1 percent None 1 TI'I-Sodllllll phosphate 2 720 10 Sodium sulfate 2 1,080 10 Acidified sodium sulfat 2 1, 080 10 Sodium chromate 5 1, 080

1 Failure is arbitrarily chosen as 50% stained.

3,271,284 Patented Sept. 6, 1966 Example 2 Galvanized sheet steel samples were pretreated in the Water solutions listed, and under the conditions stated, in Table II, as a preliminary to stain-inhibiting treatment in a water solution containing 5% by volume of waterglass and 0.5% by weight of sodium dichromate, and then tested for staining tendency by subjecting them to continuous application of a water film, with the results shown, by contrast with the stain resistance of sheets not pretreated.

TABLE II Cathodic Electrolysis Water-film Sample 1 Test, days Electrolyte Current, to failure coulombs A-l. None .i None 60 1 A=spangled; U=unspangled. 2 Same as stain-inhibiting solution.

Example 3 Galvanized steel samples were pretreated in solutions listed and under conditions stated, in Table III, to completely remove surface oxides. Half the panels were then heated in air to 1200" F. to re-form a uniform oxide film. Cathodic treatment of panels with the original oxide film and the re-formed oxide film were then treated cathodically as listed below, as a preliminary to staininhibiting treatment in the water-glass-dichromate solution previously mentioned. The results of such treatment are given in Table III:

TABLE III First Surface Re- Oxidation Final Surface Rc- Water-film duction Preheat at duction Test, days to 1,200 F. failure (a) 20 sec., 160 00 sec 20 sec., 160 amps, 142

amps, p.s.f. p.s.f. (b) 20 sec., 160 sec None 55 amps, p.s.f. (c) 20 sec., 90 sec 20 sec., 20 amps, 152

amps, p.s.f. p.s.i ((1) None..... None... None 28 (e) N one None... 20 sec? 160 amps, 118

p.s. (f) None None None 1G The reductions (except that marked were accomplished by making samples cathodic in a water solution of 3% tri-sodium phosphate, 3% sodium carbonate, and 1.5% sodium hydroxide, by weight, at F. Current densities of 160 amps. per sq. ft. of surface were used (a total of 3200 coulombs per sq. It. at each cathodic reduction).

This sample was given the second cathodic reduction in the waterglassdichromate solution above-mentioned, which represents an in situ cathodic reduction of surface oxide in the treating solution. A current density of 20 amps/ft. was used (a total of 400 coluombs/ttfl).

These data show that an oxide formed at high temperature, whether produced in the galvanizing operation or by heating after removal of the originally formed oxide, such as in trial (12), is detrimental to water film test performance. It is evident from the foregoing that the combined effect of the electrolytic cleaning of galvanized steel sheet and the subsequent chromate treatment is to improve materially the resistance to staining in storage under humid atmospheric conditions. Other incidental advantages are also realized. Sheets given our pretreatment are more easily wetted by the waterglass-dichrornate solution than sheets not pretreated.

- 3 The effect of the latter is dependent on its ability to wet the sheet surface. More reaction occurs during such treatment, furthermore, than in the treatment of sheets not subjected to our pretreatment. The improvement in solder-flow properties as a result of our invention is measurable and material, as is shown in Table IV:

TABLE IV Water-film Solder Flow Cathodic Reduction Pretreatment Test, days Test 2 height to failure of solder drop,

mils

None 60 41 3,600 eouiombs in a water solution eon- 100 18 taining 3% tri-sodium phosphate; 3% sodium carbonate and 1.5% sodium hydroxide by weight.

1 Subsequently treated with the waterglass-dichromate solution previously mentioned.

The higher the solder drop height the poorer are the solder flow properties.

Although we have disclosed herein the preferred embodiment of our invention, we intend to cover as well any change or modification therein which may be made without departing from the spirit and scope of the in vention.

We claim:

In the treatment of galvanized sheet steel to inhibit staining in storage under humid conditions by immersing it in a dilute water solution of a salt selected from the group consisting of sodium chromate and sodium dichromate, the step of preliminarily treating the sheets by electrolysis as a cathode in a dilute water solution of tri-sodium phosphate to the extent of at least 300 coulombs per square foot.

References Cited by the Examiner UNITED STATES PATENTS 1,827,204 10/1931 Mason 204-140 1,962,821 6/1934 Kochs 252-135 2,606,866 8/1952 Neish 204--140 X 2,776,255 1/1957 Hammond et a1. 204-23 JOHN H. MACK, Primary Examiner.

R. MIHALEK, Assistant Examiner. 

